The Fas (Apo-1, CD95) and Fas ligand (FasL, CD95L) system is one of the best-studied cell death systems. Fas is a type-I membrane protein abundantly expressed by cells in various tissues and particularly on activated T cells, heart cells, kidney cells and hepatocytes. FasL is a type-II transmembrane protein expressed particularly on activated T cells and natural killer cells (Nagata, S. Ann. Rev. Genet. 33:29, 1999), and is expressed constitutively in immune-privileged sites as, for example, the eye and testis (Griffith et al. Science 270:630, 1995).
Fas and FasL interactions (Fas/FasL) play an essential role in the regulation of immune cells and in the elimination of autoreactive cells (Sabelko-Downes et al. Curr. Opin. Immunol. 12:330, 2000). In addition, Fas/FasL mediates the killing of cancer cells and of virus-infected cells (Famularo et al. Med. Hypoth. 53:50, 1999; Owen-Schaub et al. Int. J. Oncol. 17:5, 2000). In contrast, FasL, expressed on cancer cells, may attack cells of the immune system (O'Connell, J. Exp. Med. 184:1075, 1996) or facilitate local tumor invasion by killing surrounding tissue (Yoong et al. Am. J. Pathol. 154:693, 1999). FasL, expressed on activated T cells, may also participate in tissue damage in fulminant hepatitis and in graft-versus-host disease (Kondo et al. Nature Med. 3:409, 1997; Braun et al. J. Exp. Med. 183:657, 1996).
Binding of FasL to Fas, or cross-linking of Fas with agonistic antibodies, induces apoptosis (Nagata, S. Ann. Rev. Genet. 33:29, 1999) that results in cell death. Apoptosis is an active cellular death process characterized by distinctive morphological changes that include condensation of nuclear chromatin, cell shrinkage, nuclear disintegration, plasma membrane blebbing, and the formation of membrane-bound apoptotic bodies (Wyllie et al. Int. Rev. Cytol. 68:251, 1980). A molecular hallmark of apoptosis is degradation of cellular nuclear DNA into oligonucleosomal-length fragments as the result of activation of endogenous endonucleases (Wyllie A. Nature 284:555, 1980). Caspases (cysteine-aspartyl-specific proteases) have been implicated as key enzymes in the execution of the late stage of apoptosis. The binding of FasL to Fas activates a cascade of caspases via a FADD adaptor (Fas-associated protein with death domain), which leads to the cleavage of various cellular substrates and to DNA fragmentation (Nagata, S. Ann. Rev. Genet. 33:29, 1999).
Synthetic oligonucleotides are polyanionic sequences that can be internalized by cells (Vlassov et al. Biochim. Biophys. Acta 1197:95, 1994) and bind selectively to nucleic acids (Wagner, R. Nature: 372:333, 1994), to specific cellular proteins (Bates et al. J. Biol. Chem. 274:26369, 1999) and to specific nuclear proteins (Scaggiante et al. Eur. J. Biochem. 252:207, 1998), and inhibit cell proliferation. Proliferation is the culmination of the progression of a cell through the cell cycle, resulting in the division of one cell into two cells. Alterations in cell cycle progression occur in all cancers and may result from over-expression of genes, mutation of regulatory genes, or abrogation of DNA damage checkpoints (Hochhauser D. Anti-Cancer Chemotherapeutic Agents 8:903, 1997).
Synthetic phosphorothioate oligonucleotides containing unmethylated CpG dinucleotides flanked by two 5′ purine and two 3′ pyrimidine (CpG motifs) are reported to induce the synthesis of cytokines by macrophages and B cells, to increase the activity of NK cells and cytotoxic T lymphocytes, and to enhance T-helper 1 response (Ballas et al. J. Immunol. 157:1840, 1996; Klinman et al. Proc. Natl. Acad. Sci. U.S.A. 93:2879, 1996; Lipford et al. Eur. J. Immunol. 27:2340, 1997). A 20 base synthetic CpG motif is reported to block Fas expression on activated B cells and to block apoptosis induced by anti-Fas monoclonal antibodies (Wang et al. Cell. Immunol. 180:162, 1997). Irradiation is reported to upregulate the expression of Fas on cancer cells (Sheard et al. Int. J. Cancer 73:757, 1997; Nishioka et al. Int. J. Mol. Med. 3:275, 1999).
The ability to modulate the Fas/FasL system has many clinical applications for use in diseases including, but not limited to, neoplastic autoimmune, degenerative and cardiovascular diseases. However, most prior Fas/FasL modulating agents have proven to be less than adequate in clinical applications. Moreover, many of these agents are inefficient, toxic or have significant adverse effects.
Therefore, there is a continuing need for novel compositions and methods that modulate the expression of Fas and FasL on cells. There is also a need for novel compositions and methods that modulate the efficacy of Fas and FasL modulatory agents on disease. There is also a need for novel compositions and methods that modulate the expression of Fas and FasL on cells in order to treat diseases in animals or humans associated with altered expression of Fas or FasL on cells.